The vibration test industry, including all major aerospace firms, automotive manufacturers, electronics companies, and the like, has adopted use of various methods and systems to simulate vibration and shock environments in order to determine their products' effectiveness and longevity when subjected to these environmental extremes. In order to obtain accurate test results, test laboratories maintain these dynamic tests in a controlled environment.
One parameter which can be important to control in vibration and shock test equipment is the axis of vibration. That is, certain test equipment will produce single-axis vibration of the article under test, while restraining any cross-axis vibration or motion which is not along the input axis of the vibration exciter.
Various types of oil film tables, bearings, bearing oil film table combinations, and flexure guidance systems are available today to support the test articles during vibration testing. One of the most successful types of vibration restraint systems available today is the hydrostatic journal bearing slip table, also known as the "Bearing Table." The first bearing table system manufactured for industrial applications was built in 1970 by Kimball Industries Inc., Monrovia, CA, in conjunction with Team Corporation, South El Monte, CA, the assignee of this application. Team Corporation designed and manufactured the internal hydrostatic journal bearings for the Bearing Table, while Kimball Industries designed and built the remainder of the granite slip table and support base. Kimball marketed the "Bearing-Line Table", as it was named, and eventually began manufacturing its own bearings based upon the original Team design, with minor variations.
The Bearing Table soon became the standard piece of equipment in the industry and was manufactured by other companies, including Ling Dynamic Systems, England; Turnkey Systems, City of Industry, CA; M-RAD Corporation, Woburn, MA; and Team Corporation. Other equipment manufacturers built similar granite slip tables with different guidance systems incorporated into the table, such as Unholtz-Dickie Corporation, Wallingford, CT, which uses low-pressure restraint guides at the corners of the table, and Ling Electronics, Anaheim, CA, which incorporates flexures at the rear of the granite table for additional mechanical guidance.
A common feature in the design of these tables is the use of a precision-ground granite slab which is lubricated with oil to create a slippery surface on which a magnesium slip plate slides during testing to support the article under test during vibration.
Although the Bearing Table has good performance, it is also characterized by the following problems which are inherent in its design:
1. A high-pressure hydraulic oil source is required to pressurize the hydrostatic journal bearings. The hydraulic power supply generally adds several thousand dollars to the cost of the table system. The power supply also can become a source of heat, which can adversely affect the table's performance.
2. Because the bearings are located at select locations within the granite table, the table's maximum load-carrying capabilities are located generally over these "bearing islands." Therefore, when the test articles are mounted to the slip plate in areas which are not supported by a bearing, the loads must be carried by the oil film/slip plate, which provides less load-carrying capacity.
3. In the event of a bearing failure, it is necessary for the manufacturer to send trained service personnel to the customer's facility to remove and replace the bearing. This requires the use of special shims and/or special machining of the bearing supports located within the granite table. This can be a costly and time-consuming procedure.
4. Most bearing table systems require porting of the magnesium slip plate to provide the bearings and granite slip surface with the necessary high-pressure oil. When smaller plates or replacement plates are used, they also must be ported, which creates additional costs. Also, the internal ports in the slip plates limit the positions of mounting inserts in the slip plate. If the inserts are installed near or on an existing oil port, the possibility of an oil leak exists, which, in most cases, requires major repairs or replacement of the slip plate.
5. When smaller slip plates are used on the tables, some of the bearings which are positioned beyond the perimeter of the small slip plates are unusable and, therefore, greatly reduce the table's performance.
6. The installation of journal bearings into a granite table requires large clearance holes in the granite for bearing clearance. These large openings reduce the table's overall load-carrying capacity.
7. The journal type hydrostatic bearings have considerable bending compliance.
The present invention provides a vibration test fixture in which a reciprocating slip plate is supported during vibration testing by a linear bearing system which overcomes the drawbacks of prior art vibration test fixtures, including the Bearing Table-type test equipment described above.